What I try to make them realize that almost always there is more than one possible path.
Online: Sure, SEO is great. But you may want to consider PPC Marketing (Google AdWords, Facebook Ads). You may consider hiring a copywriter (freelancer?) to write articles for your blog. You may want to have social media strategy. There is also an option of using public relations as a promotion method.
Offline: But you can go offline, also. Telemarketing (calling past clients, for example). Billboards placed closed to where your business is. Flyers. Mail marketing (yes, the classic one). TV, radio and newspaper ads. Networking with the right people. Organizing events with your clients and partners. Doing training for your staff.
My point is: SEO is a great tool, but you should consider a strategy, as a whole. You can work on different things, which bring different results. SEO should be part of a bigger picture. SEO is not the path, but one path.